The study of planets outside our Solar System may lead to major advances in our understanding of the Earth and may provide insight into the universal set of rules by which planets form and evolve. To achieve these goals requires applying geoscience’s wealth of Earth observations to fill in the blanks left by the necessarily minimal exoplanetary observations. In turn, many of Earth’s one-offs—plate tectonics, surface liquid water, a large moon, and life: long considered as “Which came first?” conundrums for geoscientists—may find resolution in the study of exoplanets that possess only a subset of these phenomena.

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